1. Field of the Invention
This invention relates to image processing and, more particularly, to apparatus for superimposing by electrical means a movable "pattern" on an optical image detected by a charge coupled photodiode array.
Such an apparatus has application to systems for stabilizing optical images and for extracting information for use in tracking various moving objects, including information regarding the presence, the position, and the velocity of objects.
2. Description of the Prior Art
Considerable prior art exists in the field of image processing. The prior art may conveniently be divided into categories such as spatial filtering, image position measurement, and image velocity sensing.
Spatial filtering may be used when it is feasible to distinguish images of different sizes from one another. For example, a spatial filter consisting of a series of opaque grids could be used as a high pass optical filter, to average out large images while passing small images having higher spatial frequencies. Spatial filters are used in a variety of instruments including the position and velocity sensors described below.
U.S. Pat. No. 2,941,080, issued June 14, 1960 to S. Hansen shows apparatus used for determining the position of an image in a field of view. In that invention, the image of a star or other object of small angular extent is chopped by a reticle pattern consisting of two sets of grids having different spatial frequencies. Each grid pattern is interrupted at intervals to generate a succession of bursts of high frequency pulses. The phase of the bursts of pulses is related to the image position, and the frequency of the pulses within the burst facilitates the use of tuned electronic filters in alternating current signal processing circuits.
The use of two superimposed grid patterns having different spatial frequencies results in two distinct sets of pulse repetition frequencies within the burst of pulses that are generated. These two carrier frequencies, associated with the X and Y directions in the field of view, permit narrow band electrical filtering to separate the position coordinates in the two dimensions, simultaneously.
The reticle in Hansen was rotated through a gear train driven by an electric motor, and a reference signal was generated by a potentiometer or other sensor geared to the driving mechanism. A single photodetector converted the light from the chopped image into an electrical signal.
In U.S. Pat. No. 3,500,050 issued Mar. 10, 1970, to Hillman, and assigned to the assignee of the present invention, an image is focused on a portion of a spoked reticle and simultaneously, light from a non-moving source, for example a light-emitting diode, is also focused on the reticle. Separate photodetectors detect the chopped light from these sources.
If the image is moving in the direction of the spokes of the chopper, the doppler effect causes a reduction in the chopping frequency. This reduction in frequency may be used as a basis for determining the velocity of the image in the direction of the spokes of the reticle. From a mechanical standpoint this type of image motion sensor does not differ greatly from the image position sensor of Hansen described above. A reticle having alternate opaque and transparent spokes is rotated in the image plane by an electric motor and possibly a gear train.
Electro-optical instruments in which a reticle physically moved in the image plane are generally thought of as precision instruments. The reticle is mounted accurately with its axis of rotation truly perpendicular to the plane of the image. Care is taken to assure that the geometric center of the reticle lies on its axis of rotation. Also, the reticle and the apparatus used to rotate it may be carefully balanced both statically and dynamically, to reduce mechanical vibrations to an insignificant level.
In the present invention, the moving reticle is generated electronically and the advantages of the prior art precision instruments are attained but without the need for moving parts.